After A While, My Mac Stops Working

Question

I know that's vague, but that's what is happening.
If I leave my system alone after a reboot, about 4 hours later, the system starts acting strange. If I attempt to eject the DVD drive, the system sort of hangs. It is responsive, but the eject menu item beach balls. Ejecting from the Finder, dragging to the trash and pressing the eject key all yield the same result.
If I attempt to open System Preferences, the icon appears in the Dock and the blue light is on underneath it, but that is it. Force Quit will not get it out of the Dock. The process is listed via "ps -ef", but killing it does nothing.
Lastly, if I try to reboot or shutdown, the Dock and menu dissapear and the shutdown spinner is visible, but it will never shut down. I have to hold in the power button to shut it off.
After it reboots, everything works fine. It's only after about 4 hours of idle time that his happens.
System Specs:
PowerMac Dual 1.8GHz G5
2GB of RAM (8 slots on motherboard)
Dual 500GB SATA drives as a RAID 1 Root
Mac OS X 10.5.5

Answer 1

I use RAID a lot, but I also tend to keep multiple backups; and, rebuild arrays much more frequently than most. Often when there are changes made under the hood so to speak, and definitely when OS X has gotten updates (sometimes even mid-stream like 10.4.4 made some changes to how the OS mounts and communicates to disks or handles SCSI).
Leopard 10.5.0 and again with 10.5.2, introduced some changes, and affects some external or PCI controllers and how drives are mounted. In some cases ATTO and Firmtek had to issue updates or tell customers to hold off. Controllers are very likely to need drivers and updates. Even if you aren't using a controller, just something to be aware of.
And ideally your RAID is composed of identical pair of drives. I've never cared for mirror, after trying it a couple times, and then mostly with SoftRAID. And do make sure you have it backed up, some don't. But never caused an issue with not waking from sleep or freezing either. That seemed to be from USB or FW devices, or PCI controllers.